Along with communication demand increases, an optical network utilizing a wavelength division multiplexing (WDM) is widely used. The wavelength division multiplexing is a technology for transmitting by multiplexing a plurality of optical signals having different wavelengths.
According to the wavelength division multiplexing, for example, the optical signal of a transmission rate of 40 (Gbps)×88 waves is multiplexed and is capable of transmitting as a wavelength multiplexed optical signal (hereinafter, referred to as “multiplexed optical signal”). As a wavelength multiplexing transmission apparatus utilizing the WDM, for example, a Reconfigurable Optical Add-Drop Multiplexer (ROADM) apparatus is known.
About a spectrum of the multiplexed optical signal, the optical signal has a certain wavelength interval (wavelength spacing) such as 50 (GHz) or 100 (GHz). The wavelength interval is referred to as an ITU-T grid (ITU-T: International Telecommunication Union Telecommunication Standardization Sector (International Telecommunication Union)) and the like, and is widely used for the wavelength multiplexing transmission apparatus.
In the related art, as future demand for communications is expected to be increased, a multi-value modulation system such as Dual Polarization (DP)-Quaternary Phase-Shift Keying (QPSK) used for wireless communication is applied to the wavelength multiplexing transmission apparatus and coherent transmission is attempted to be realized. Thus, in the wavelength multiplexing transmission apparatus, it is preferable that the optical signals having various communication capacities different in the modulation system as well as the transmission rate be accommodated in the multiplexed optical signal.
Thus, a flexible grid technology in which the optical signals having various bandwidths are flexibly accommodated in the multiplexed optical signal by varying the wavelength interval is developed. For example, the flexible grid technology is defined in the ITU-T recommendation G. 694. 1. According to the flexible grid technology, different from a case where a fixed wavelength interval such as the ITU-T grid is used, it is possible to set the wavelength interval between the optical signals in which the spectrum is adjacent based on the minimum bandwidth depending on the type of the optical signal. Thus, the transmission capacity for each optical fiber increases and wavelength accommodation efficiency is improved.
However, for example, if the optical signal during operation is replaced with another optical signal having a different bandwidth, an unused fragmentation area occurs between the spectra of adjacent optical signals by a difference in passband widths of the optical signal before and after the replacement. Thus, there is a problem in that the wavelength accommodation efficiency of the optical fiber is decreased by increasing in the fragmentation area as the replacement of the optical signal progresses.
Meanwhile, for example, in Kyosuke Sone, et al. “First Demonstration of Hitless Spectrum Defragmentation using Real-time Coherent Receivers in Flexible Grid Optical Networks”, ECOC 2012 and in F. Cugini, et al. “Push-Pull Technique for Defragmentation in Flexible Optical Networks”, JTh2A, OFC2012, an uninterrupted defragmentation technology is disclosed in which the fragmentation area is reduced by synchronizing a wavelength of wavelength variable laser of a transmission node with a passband of a wavelength filter of a relay node to be changed.
Furthermore, about wavelength control of the optical signal, for example, in Japanese Laid-open Patent Publication No. 2012-195787, a configuration is disclosed in which the wavelength is arranged such that the optical signals having the same modulation system are adjacent to each other.